1. Field of the Disclosure
The present invention relates generally to fuel cells. More particularly, the present invention relates to fuel cells utilizing flow-through electrodes and alkali electrolytes.
2. Background of the Disclosure
In general terms, a fuel cell generates DC electricity by a chemical reaction or reactions occurring at an anode and a cathode. The electricity is utilized by an electrical circuit communicating with the fuel cell. The fuel cell utilizes an electrolyte that essentially functions to transport electrically charged particles from one electrode to the other. Liquid electrolytes such as alkali, molten carbonate, and phosphoric acid, and solid electrolytes such as proton exchange membrane (PEM) and solid oxide have been employed. The fuel cell may also utilize one or more catalysts to promote the proper reactions. A basic fuel for the reactions is hydrogen, hydrocarbons, alcohols, or the like that can be oxidized. In addition, oxygen is generally reduced at the cathode. A primary byproduct is water, but conventional fuel cell designs inevitably yield several more undesired byproducts such as carbon monoxide, sulfur, and the like, particularly when employing a carbon-based fuel. Different types of fuel cells have been developed and are well-known. Generally, each type has well-known advantages and disadvantages.
In the operation of a typical fuel cell, oxygen is applied at the cathode and the fuel is applied at the anode. In some types of fuel cells, the oxygen combines with electrons returning from the electrical circuit and hydrogen ions that have traveled through the electrolyte from the anode. In other types of fuel cells, the oxygen reacts with water and electrons to form hydroxyls, which then travel through the electrolyte to the anode, where it combines with hydrogen ions. The electrolyte serves as the vehicle by which appropriate ions are transported between the anode and cathode. In alkali fuel cells, the electrolyte transports hydroxyl ions from the cathode to the anode. In molten carbonate fuel cells, the electrolyte transports carbon trioxide ions from the cathode to the anode. In solid oxide fuel cells, the electrolyte transports oxygen ions from the cathode to the anode. In phosphoric acid and PEM fuel cells, the electrolyte transports hydrogen ions from the anode to the cathode. At the anode or cathode, depending on design, hydrogen and oxygen ions combine to form the water. The fuel cell will continue to generate electricity as long as hydrogen, oxygen, and electrolyte are available for reaction and the catalyst is not too degraded.
FIG. 1A illustrates a common configuration for a membrane electrode assembly 10 known in the prior art. Membrane electrode assembly 10 consists of a proton-exchange membrane (PEM) 12 that serves as the electrolyte and as a support structure for an anode 14 and a cathode 16. The magnified view of FIG. 1B depicts the micro-scale composition of the electrodes (anode 14 and cathode 16). The composition consists of a mixture of finely divided, widely dispersed, very small (typically micro-scale) particles of platinum dust 22 supported on, and adhered to, larger but still microscopic carbon particles 24. This substrate/catalyst complex is mixed with an adhesive binder to hold it together, fibers 26 (typically carbon) to increase structural integrity, and hydrophobic PTFE 28 to help the egress of water. This mixture is applied to a structural support that, as indicated for the case of a PEM-based fuel cell, is often the electrolyte membrane 12 itself.
The current design as exemplified by membrane electrode assembly 10 illustrated in FIGS. 1A and 1B is functional and an improvement over earlier designs, but it is still embodies significant limitations. Widely recognized problems attending this design include the following. Only a small fraction of the mass of the catalyst (e.g., platinum dust 22 in FIG. 1B) is available for catalytic activity. The catalyst molecules on the surface of the catalyst particle are the only part that touches the fuel. The rest of the mass of the catalyst is trapped in the interior of the particle where it cannot touch the reactants. The extent to which the expensive catalyst is wasted in the above manner can be estimated as follows. Assuming that the particles are roughly spherical, the formula for the volume of a sphere is V=4/3πr3, and the formula for the surface area of a sphere is SA=4πr2. Accordingly, the ratio of surface area to volume of a sphere is SA/V=3/r. The radii of typical catalyst particles for PEM cells are 12.5-24.5 Å. Therefore, in PEM cells, the ratio of surface area-to-volume ranges from 1:4 to 1:8, with 1:6 as a median. It follows that only about ⅙th or 17% of the platinum is potentially available as a catalyst, such that roughly 83% of the mass is wasted.
The degree of waste is much worse in direct methanol fuel cells, which require larger particles and heavier catalyst loadings. Direct methanol cells generally employ particles with radii of 44-125 Å. Plugging these numbers into the above equations reveals that the surface area-to-volume ratios vary from 1:15 to 1:41 with a median of 1:28. Therefore, only 3.6% of the surface area of catalyst is available for catalytic activity. The remaining portion of the catalyst is potentially wasted, for instance, by being locked inside the particle. Moreover, part of the 3.6-17% of platinum potentially available for catalytic activity is additionally lost to the reaction because it is in intimate contact with inert substrate materials such as PTFE, glues, structural fibers, or the carbon support itself (see, e.g., FIG. 1 and accompanying discussion above). This contact area excludes contact with the reactants and thus further limits the usable, catalytic surface area. Losses from these contact areas can be conservatively estimated to be about an additional 5%, but are usually functionally much higher.
Additionally, a yet additional loss of catalytic utilization occurs because the reaction can only take place at the triple-interface of the fuel, electrolyte, and catalyst. Only a fraction of the catalytic mass left after the above reductions encounters the electrolyte/fuel interface. Even in the case of the particles that do touch the interface of fuel and electrolyte, not all of the surface area experiences the interface. A significant amount of the catalyst does not even touch the electrolyte, and therefore does not participate in generating electricity. Furthermore, those particles that do experience a favorable interface produce or use water, thereby changing the fuel/water ratio in their immediate microenvironment, often decreasing catalytic efficiency.
Another problem is that the materials and mixtures utilized in the prior art are fairly electrically resistant. This internal resistance substantially decreases electrical production efficiency and necessitates the use of conductive current-collecting “field flow” plates that add sizeable cost and volume to the fuel cell or fuel cell stack. Currently designed field-flow plates also contribute to considerable internal resistance.
In addition, the carbon-platinum mixture utilized in prior art approaches is essentially a brittle composition of dust or a composite of powders. The mixture is sensitive to vibration and mechanical, electrical and thermal stresses. Over time the mixture tends to disintegrate and thereby limit lifespan and efficiency.
Moreover, in current fuel cell designs, the fuel flows by and diffuses into the anode but not through it. Consequently, inert compounds can build up in the pores and physically block the fuel from reaching the catalyst, hence further limiting efficiency.
Still further, the individual parts of the fuel cell stack of current technology need a uniform, fairly exact degree of humidification in order to properly function. Water is produced at one electrode and thus can potentially flood the electrode. Water is used up at the other electrode, thereby drying that electrode, and is dragged away by concentration gradients and electro-osmotic forces. To ameliorate or compensate for these flooding and drying events, current fuel cell designs must resort to the addition of extensive, costly and power-robbing balance-of-plant apparatus. Generally, balance-of-plant apparatus is the ancillary equipment necessary for supporting operation of a fuel cell and conditioning its outputs to usable forms. Balance-of-plant apparatus can include fuel stock scrubbers, controllers, heat exchangers, fuel reformers, shift reactors, humidifiers, dehumidifiers, pumps, compressors, regulators, power conditioners, tanks, valves, pipes, hoses, sensors, thermal regulators, manifolds, filters, and the like. In conventional fuel systems, the massive balance-of-plant apparatus can dwarf the actual fuel cell stack. Much of the prohibitive cost of fuel cell production can be attributed to the balance-of-plant and not the fuel cell stack itself. Unfortunately, even with the use of balance-of-plant apparatus, the simultaneous ideal humidification for each electrode (anode and cathode, as well as the electrolyte) is never quite uniformly achieved.
Poisoning and contamination remain a pervasive problem in many fuel cell designs. Catalysts suffer from poisoning by common contaminants found in many fuel stocks. They are even poisoned by intermediary compounds produced from their own reactions. Poisons can include, for instance, a variety of sulfur- and carbon-based compounds. Of special significance is carbon monoxide as it is a common intermediary compound of carbon-based fuels such as methanol. Over time, these substances adhere to the catalytic particles of conventional electrodes, degrading their performance and limiting their lifespan. Sensitivity to poisoning seriously limits the feasibility and commercial viability of the currently existing technologies. It is especially a problem in fuel cells that use currently available fossil fuels and natural gas derivatives. These fuels have relatively high amounts of sulfur compounds and complex hydrocarbons that form a variety of toxic intermediary compounds. To partially atone for this problem, manufacturers are forced to incorporate expensive, additional balance-of-plant apparatus such as those noted above, particularly fuel stock scrubbers, reformers, shift reactors, and advanced filters, all of which can be bulky and/or expensive and escalate inefficiency, maintenance requirements, and pollution. The extra equipment also requires energy to run that is parasitically drawn from the output of fuel cell. The costs attending this additional equipment can obviate the advantages they provide such as the ability to use readily available fuel stocks from the current infrastructure.
In view of the foregoing, a widely recognized need exists for ongoing improvements in fuel cells and fuel cell systems, and components thereof. In particular, a continuing need exists for increasing operating efficiencies and extending the useful life of materials and components, as well as reducing the costs, complexities, and components required for providing commercially viable fuel cells and systems.